In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine, which turns the shaft and provides power to the compressor and to the fan. In a more complex version of the gas-turbine engine, the compressor and a high-pressure turbine are mounted on one shaft having a first set of turbines, and the fan and a low-pressure turbine are mounted on a separate shaft having a second set of turbines. The hot exhaust gases and the air propelled by the fan flow from the back of the engine, driving it and the aircraft forward. The hotter the combustion and exhaust gases, the more efficient the operation of the jet engine. There is thus an incentive to raise the combustion-gas temperature.
The turbine (sometimes termed a “turbine rotor”) includes one or more turbine disks, a number of turbine blades mounted to the turbine disks and extending radially outwardly therefrom into the combustion-gas flow path, and rotating seals that prevent the hot combustion gases from contacting the turbine shaft and related components. The materials used in the turbine limit the maximum operating temperature of the combustion gas. Great efforts have been made to increase the temperature capabilities of the turbine blades, resulting in increasing combustion as operating temperatures and increased engine efficiency.
As the maximum operating temperature of the combustion gas increases, the turbine disk and seals are subjected to higher temperatures in the combustion and bleed gas environment. As a result, oxidation and corrosion of the turbine disk and seals have become of greater concern. Alkaline sulfate deposits resulting from the ingested dirt and the sulfur in the combustion gas are a major source of the corrosion, but other elements in the aggressive combustion and bleed gas environment may also accelerate the corrosion. The oxidation and corrosion damage may lead to premature removal and replacement of the turbine disk and seals unless the damage is reduced or repaired.
The turbine disks and seals for use at the highest operating temperatures are made of nickel-base superalloys selected for good toughness and fatigue resistance. These superalloys are selected for their mechanical properties. They have some resistance to oxidation and corrosion damage, but that resistance is not sufficient to protect them at the operating temperatures that are now being reached.
One corrosion resistant coating, which has been developed for use with turbine disks and seals, is a three-layer oxide based paint system that may be applied to the corrosion susceptible areas of turbine disks and seals. This three-layer oxide based paint coating system comprises alumina and chromia particles, which are oxides of metals that are found in the superalloy substrate. Such a coating system does not adversely affect the fatigue life of the turbine disks and seals and greatly increases the corrosion resistance of the component to which it is applied. However, during the application of such paint systems during initial manufacture of aircraft engine components, small coating imperfection regions sometimes develop, exposing the superalloy substrate. Placing an aircraft engine into service with such coating imperfections present on the component would open up the component under these areas of imperfection to corrosion. One solution is to strip all of the coating off of the component and reapply the coating. However, such a solution is expensive and time consuming.
What is needed is a method of applying touch-up oxide paint onto the coating imperfections of layer paint oxide coating systems on turbine disks and seals. Such touch-up oxide paint should be able to be applied after initial manufacture and inhibit corrosion from occurring between planned engine overhauls, which generally occur approximately every 3000 cycles. In addition, such application of touch-up oxide paint should not affect the low cycle fatigue (LCF) life, readily adhere to a superalloy substrate, and function effectively to 1400° F. (760° C.). This need extends to other components of the gas turbine engine with layer paint oxide coating systems as well. The present invention fulfills these needs, and further provides related advantages.